1. Field of Invention
This invention relates to non-contact ultrasonic thickness measurement of sputtering targets bonded to a backing plate using immersion bubbler technique and data acquisition over-sampling.
2. Description of Related Art
In the fabrication of integrated circuits and other electronic, opto-electronic, microwave, and MEM devices, multiple deposition and etch processes are performed in sequence to fabricate the desired electronic structures or devices. The current trend in fabrication has been to improve the performance and reliability of devices with simultaneous reduction in manufacturing cost. The ultimate goal is to fabricate devices in a way that combines improved performance (speed and capacity), with improved cost efficiency of manufacturing process. Manufacturing cost can be kept under control in a number of ways, particularly by reducing the cost of consumables used in the process. One of such consumables is a sputtering target. The cost of the sputtering target can be reduced substantially by replacing the part of expensive target material which is not a part of the sputter erosion process, with less expensive commercially available “backing” material. The “backing” material, in addition to cost reduction, provides improved mechanical, thermal, and even electrical properties of the target. This becomes of particular importance for targets made of mechanically soft materials. These targets can be deformed by thermo-mechanical stresses applied to the target during sputter-related heat load cycling. In contrast, a backing plate made of “backing” material provides extra mechanical stiffness and improved thermal conductance.
Backing material can be attached to the target in a number of ways. However, only three techniques, namely, mechanical, diffusion, or solder bonding, are of practical interest for target-to-backing plate joining. All three techniques require high or elevated pressure and high or elevated temperature to complete the bonding process. The drawback of these techniques is the significant difficulty in maintaining the pre-designed shape of the bond interface, for example, the flatness for planar targets. In many cases, when different target and “backing” materials are used, mismatches in thermal expansion coefficients of the different materials causes the bond interface to deflect from an originally predefined shape. The mechanical flattening which usually follows the bonding process is thus not always capable of flattening the bond interface to a satisfactory level. Therefore, in many cases only a partial correction of deflection of bonded interface is achieved that results in target thickness variations all over the target after sputter surface machining. The thickness variations, in turn, require close monitoring and measuring. Failure to determine the target minimum thickness may result in catastrophic performance of the target when the target sputters through the bond interface into the backing plate, causing contamination in sputtered films.
Attempts to use designing means to change the shape of pre-bonded surfaces to compensate for bonding-related deflection has shown mixed results. On the other hand, modeling, for example, by using finite element analyses, does not provide a satisfactory prediction for bond interface deflection due to many uncontrolled variables, which are typically not accounted for during analysis.
Therefore, there remains a need to measure the actual thickness of the target between front surface and the bonded interface. The conventional technique for thickness measurements of bonded assemblies is the ultrasonic NDT. A number of portable and stationary thickness measurement instruments or gauges are available from many NDT equipment manufacturers. The typical ultrasonic thickness gauge comprises an ultrasonic piezoelectric transducer electrically connected to an electronic block comprising, in turn, a pulser, a receiver, and a signal processor, which are controlled by the gauge's internal microcontroller.
The transducer, when excited by a short electric pulse from the pulser, generates a burst of high frequency mechanical vibrations or sound waves. This sound burst or pulse propagates through the specimen if the specimen is ultrasonically coupled to the transducer. The sound pulse, when it reaches the bond interface, bounces back to the transducer in the form of an echo. The transducer converts the echo back into an electric signal. The electric signal is processed by the gauge, which calculates the thickness of the specimen. When the thickness is calculated it is displayed and transferred to the remote controller if the gauge is equipped with a serial, USB, or other type of port.
Typical ultrasonic thickness gauges operate in the “Pulse/Echo” mode, by timing precisely the reflection of the echo bounced back at normal incidence from the reflecting surface such as the bond interface. If the gauge is calibrated to the speed of sound in the test material then the thickness is determined by an internal calculation performed by the gauge processor using the following relationship [Ref. 1]:Thickness=V(t−t0)/2where: V—the velocity of sound in the material,                t—the measured transit time of sound pulse,        t0—the zero offset factor (to correct for transducer internal delay, cable delay, and other fixed delays).        
A typical gauge can measure thickness in three modes. Mode 1 is used with contact transducers when the transducer is directly coupled to the surface of the specimen. In this mode, the transit time is measured between a main bang MB pulse and a first returning echo. This method is simplest and it is frequently used for manual thickness measurements when the specimen is relatively thick, and only a few thickness data points are required to collect. Modes 2 and 3 are used with delay line, or immersion, transducers for the specimens of any, but preferably small or moderate, thickness when improved measurement accuracy is required. In Mode 2, the transit time is measured between the front surface and the first backwall (or bond interface) echoes, while in Mode 3 the transit time is usually measured between two consecutive echoes following the front surface echo. It is important to know that Mode 2 is preferred for materials with a higher sound attenuation, such as copper, cobalt, tantalum, or WTi while Mode 3 (which is most accurate among all three modes) is preferred for low attenuated materials such as aluminum, titanium, or tungsten.
Implementation of Modes 1, 2, or 3, when the transducer (with or without delay line) is directly coupled to a target surface, is limited to scratch resistant materials, since only a droplet of water can be used for target coupling. As seen frequently in practice, a thin layer of water does not provide an adequate protection for target surface, particularly of soft materials such as aluminum or copper, from scratching. Another drawback of direct contact coupling is that manual operation depends on operator hands-on experience. A still further drawback of direct contact coupling is the occasional difficulty in finding a region of the target with a minimal thickness. However, the direct contact coupling has one important advantage, namely, compactness and mobility, that makes it a preferred technique for use in-situ when the part remains attached to the chuck of the machining tool. This simplifies testing and reduces the overall test time.
Non-contact immersion Modes 2 and 3 are designed to overcome limitations of contact methods providing non-scratching, accurate, and automated methods of testing. Immersion thickness testing can be done in two ways. It can be done by submerging the entire target assembly and the transducer into a tank with de-ionized (DI) water where a stationary column of coupling water between target and transducer is formed. The advantage of this technique is the ability of using conventional C-Scan technology and equipment. The disadvantage of this technique is the relatively high cost since several steps are required to complete the test. Steps include removing the target from machining tool and placing it into a C-Scan tank for testing, then replacing the target back to the machining tool to complete machining. After-test machining is also required, at least as a refinishing measure, to remove the hydro-oxidation caused by extended exposure of the target surface to the water. Aluminum and copper-made targets are among of most susceptible to hydro-oxidation.
The other way of using immersion testing is a bubbler technique. The bubbler technique may provide a definite advantage for target thickness measurement compared to all previously discussed methods. The sound beam, in this case, propagates through a column of flowing water, which impinges into the target surface. As a result, the water exposure and subsequently hydro-oxidation can be minimized drastically by reducing the size of the water contact area and exposure time. This can be achieved by decreasing the diameter of the water contact area and by bringing this area into a continuous moving contact all over the target surface. However, there is a limitation frequently imposed by conventional bubbler techniques. The limitation is a lack of spatial resolution. Conventional conveyor-based bubbler techniques, for example, used in metal rolling mills and etc., acquire thickness data at certain spaced intervals usually pre-defined by conveyor speed and data acquisition rate. For high conveyor speed applications a plurality of positions from where the thickness data are sampled, can be separated be lengthy intervals that pose a danger of missing the positions with a critical minimum thickness. This is absolutely not acceptable for sputtering target applications. The region of a target with a minimum thickness should always be detected since the minimum thickness is among the most critically controlled target geometrical parameters, which governs pass/fail criterion of the target. Conventional bubbler techniques have another drawback, which may interfere with test remote operation. This additional drawback is the possibility of interruption in the data acquisition process due to occasional discontinuity in the water flow, especially for small bubbler apertures when a chain of air bubbles is formed in the water supply stream.
Therefore, there is still a need in the art for precision, low cost, non-contact, automated, ultrasonic target thickness measurement technique performed in-situ inside a machining tool.